Earths next mass extinction
Contents
<p>In 1980, the physicist Luis Alvarez and his geologist son Walter published a paper arguing that the dinosaurs had been killed by a rock from space. Their evidence was a thin, worldwide layer of clay unusually rich in iridium — an element rare in Earth’s crust but common in asteroids. The claim was ridiculed for a decade until, in 1991, a 180-kilometre crater buried beneath the Yucatán Peninsula near the town of Chicxulub was confirmed as the impact site. The Alvarez hypothesis had named the murder weapon for the most famous mass extinction in history. What makes that story unsettling today is not the asteroid. It is that palaeontologists now count five of these catastrophes in the fossil record — and increasingly argue that we are living through the sixth, this time with no rock and no volcano to blame.</p>
<h2 id="what-a-mass-extinction-actually-is">What a mass extinction actually is</h2><div class="ad-unit ad-in-article" aria-label="Advertisement">
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<p>A mass extinction is not merely a bad century for wildlife. Geologists reserve the term for events in which a large fraction of all species — commonly more than three-quarters — vanishes in a geologically short window, sometimes tens of thousands of years, sometimes a few hundred thousand. Five such events stand out clearly enough in the rock to have earned names, the “Big Five” first formally catalogued by palaeontologists Jack Sepkoski and David Raup in a landmark 1982 analysis of the marine fossil record. Each has a different killer, and reading them in order is a lesson in the many ways a planet can turn on the life it carries.</p>
<h2 id="the-end-ordovician-ice">The End-Ordovician: ice</h2>
<p>The oldest of the five struck about 444 million years ago, at the end of the Ordovician period, when complex life was still confined to the seas. A pulse of global cooling locked huge volumes of water into ice sheets, sea levels dropped, and the shallow coastal habitats where most species lived drained away. Roughly 85 percent of species disappeared. It is the only one of the Big Five in which cold, rather than heat, did most of the killing — a useful reminder that it is rapid change in either direction, not warmth alone, that ecosystems cannot survive.</p>
<h2 id="the-late-devonian-and-the-great-dying">The Late Devonian and the great dying</h2><div class="ad-unit ad-in-article" aria-label="Advertisement">
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<p>The Late Devonian extinction, around 372 million years ago, unfolded as a drawn-out series of pulses rather than a single blow, and it hit reef-building organisms especially hard; coral reefs would not fully recover for over 100 million years. But the event that dwarfs all others came at the end of the Permian, roughly 252 million years ago — the Permian-Triassic extinction, nicknamed “the Great Dying.” It remains the worst catastrophe life has ever endured, erasing an estimated 90 to 96 percent of marine species and around 70 percent of land vertebrates.</p>
<p>The cause was not a rock from space but the Earth itself. The Siberian Traps, one of the largest volcanic events in the planet’s history, erupted across what is now Siberia, flooding an area the size of Western Europe in lava and pumping vast quantities of carbon dioxide into the atmosphere. The result was runaway warming, acidified and oxygen-starved oceans, and an environment so hostile that recovery took an estimated 10 million years. The Great Dying is the ancient event that most closely rhymes with the present one, because its mechanism — a sudden flood of carbon into the air and sea — is the mechanism humans are reproducing.</p>
<h2 id="the-end-of-the-triassic-and-the-age-of-dinosaurs">The end of the Triassic, and the age of dinosaurs</h2>
<p>The fourth event closed the Triassic period about 201 million years ago. For years its cause was debated, but the evidence now points firmly to another burst of volcanism: the Central Atlantic Magmatic Province, the eruptions that accompanied the tearing-apart of the supercontinent Pangaea as the Atlantic Ocean began to open. As with the Great Dying, the killer was carbon dioxide and the warming it drove, which studies estimate at 3 to 6 degrees Celsius. The extinction cleared the ecological stage of many competitors — and it was into that emptied world that the dinosaurs expanded to become the dominant land animals for the next 135 million years.</p>
<h2 id="the-asteroid-that-ended-their-reign">The asteroid that ended their reign</h2>
<p>The fifth and most familiar, the Cretaceous-Paleogene extinction, arrived 66 million years ago in the form of the Chicxulub impactor — the very object the Alvarezes had inferred from a layer of iridium. The strike itself was survivable at a distance; the aftermath was not. Wildfires, a sky darkened for years by debris, collapsed food chains, and a plunge in temperatures killed roughly three-quarters of all species, including every non-avian dinosaur. The birds that fly today are the dinosaurs’ living descendants, and the mammals that had scurried in the undergrowth for over 100 million years finally inherited a world without giant reptiles pressing down on them. Every human alive owes their existence to that catastrophe.</p>
<h2 id="the-sixth-and-who-is-causing-it">The sixth, and who is causing it</h2>
<p>Which brings us to now. The International Union for Conservation of Nature maintains the Red List of Threatened Species, and the numbers it reports describe an event without precedent in the fossil record: current extinction rates are estimated by many biologists to run tens to hundreds of times above the natural “background” rate, and a 2020 study by Living Planet analysts recorded an average decline of around 68 percent in monitored vertebrate populations between 1970 and 2016. Insects, the invisible machinery of most ecosystems, are collapsing in regions where they have been carefully counted, such as the German nature reserves where a 2017 study found flying-insect biomass had fallen by more than 75 percent in 27 years.</p>
<p>The mechanisms are habitat destruction, hunting, invasive species, and — increasingly — the carbon-driven warming that also powered the Great Dying and the end-Triassic. The difference is the cause. Every previous mass extinction was triggered by something without agency: an ice age, a volcano, a rock. This is the first in which the trigger can read about itself, understand the geology, and choose whether to continue. That capacity is not shared by the microbes and viruses that shape life in quieter ways, as the emergence of pathogens like the <a href="/story/marburg-virus/">Marburg virus</a> reminds us — nature reshuffles the living world constantly, but it does not deliberate.</p>
<p>There is one more difference worth stating plainly. Every earlier mass extinction played out over tens of thousands to hundreds of thousands of years — slow enough that, from inside any single lifetime, nothing would have looked wrong. The Siberian Traps did not erupt in an afternoon; the Great Dying unfolded across a span longer than the entire history of our species. The sixth extinction is compressed into decades. The 68 percent average decline in monitored vertebrate populations happened within the lifetime of people now reading about it, which makes this the first extinction event that a single human observer can watch progress from beginning to middle. That compression is both a horror and, potentially, an opportunity: change fast enough to notice is also change fast enough, in principle, to interrupt.</p>
<h2 id="whether-the-sixth-can-be-halted">Whether the sixth can be halted</h2>
<p>The honest answer is that some loss is already locked in; species now vanishing cannot be recovered. But “mass extinction” describes a spectrum, not a single fixed outcome, and the difference between losing a fifth of species and losing three-quarters is enormous and still, in part, a human choice. Slowing warming, protecting habitat, and curbing the pressures on the most vulnerable populations all measurably change the curve. The comparison worth holding onto is not with the asteroid — which no one could have stopped — but with the volcanic carbon events, whose slow, self-inflicted quality most resembles the crisis we are steering. The same runaway-carbon logic runs through Earth’s whole <a href="/story/the-climate-change-chronicles-a-whirlwind-tour-through-earths-wacky-weather-history/">wild and volatile climate history</a>, and knowing that history is the difference between reacting blindly and acting with foresight.</p>
<h2 id="fun-facts">Fun facts</h2>
<ul>
<li>The iridium layer that solved the dinosaur mystery is thin enough to hold in your fingers yet appears in the rock record on every continent — a single bad day preserved worldwide.</li>
<li>After the Great Dying, so much life was wiped out that the recovery period is sometimes called the “coal gap”: for millions of years there was too little forest to form significant coal deposits.</li>
<li>Birds are surviving dinosaurs, which means the Cretaceous extinction did not end the dinosaurs’ reign so much as prune it — there are more species of living dinosaur (birds) today than of mammals.</li>
<li>The Chicxulub crater is invisible at the surface; it was found by geophysicists prospecting for oil, who noticed a buried ring structure long before anyone connected it to the extinction.</li>
<li>Mammals existed alongside dinosaurs for over 150 million years before the asteroid; they did not out-compete the dinosaurs so much as outlast the disaster that removed them.</li>
</ul>
<h2 id="a-closing-reflection">A closing reflection</h2>
<p>There is a strange consolation buried in the geology: the Earth has done this five times and, each time, life has come roaring back in forms no one could have predicted. The planet does not need us to recover. What is genuinely new about the sixth extinction is not its scale, which does not yet approach the Great Dying, but its authorship — for the first time the agent of the catastrophe can see the layer of clay it is laying down for some future geologist to puzzle over. Whether that self-awareness becomes a warning heeded or merely a more detailed epitaph is the one variable no previous extinction had.</p>
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